I got the impression that the bookend scenes would be set towards the end of Return of the King, around the scene where Frodo is writing in the Red Book in his study in Bag End. The earlier they set the scene, the harder it will be to make Elijah Wood look the right age. I suppose they could set the scenes before Lord of the Rings if they really wanted to, but for the purposes of my speculations I assumed that the scenes would be after Frodo's quest.